The invention particularly relates to radiation sensors and here preferably radiation sensors designed to measure temperatures by means of an infrared radiation measurement. The actual sensor elements are, in this case, those structures onto which, concretely, the infrared radiation to be measured is incident and which translate it into electrically usable signals, for example a current, a voltage, an electric charge, in a certain manner. They may be thermopiles or pyrodetectors or bolometers.
Particularly thermopiles have the property that their electrical output signal does not only depend on the incident electromagnetic radiation (in the infrared range) but also on the ambient temperature of the environment in which the sensor element works. To disconnect the sensor elements of heat sinks and of the ambient temperature as much as possible thermopiles are arranged on thermally poorly conducting structures as shown, for example, in FIG. 1. The actual sensor element is denoted by the numerals 4 (4a, 4b). It has a hot end 4a and a cold end 4b. Above the hot end 4a an absorption layer 5a may be provided which may, for example, be of a dark colour so that it absorbs the incident infrared radiation (denoted by IR(To)) particularly well and therefore leads to the heating of the hot end 4a. On the other hand, a reflective layer 5b may be disposed above the cold end 4b, said reflective layer 5b reflecting the incident infrared radiation so that the cold end 4b heats up correspondingly less. The temperature difference between the cold and the hot end results in a measurable voltage difference. A plurality of said structures may be connected in series so that a correspondingly higher signal voltage is obtained.
The sensor elements are disposed on a thin membrane 3 which itself is supported by a substrate 1 formed as a frame. The hot ends 4a are, in this case, regularly not disposed above the frame 1 but on the membrane 3 above the orifice 2 of the frame 1. In this way the hot end 4a is thermally separated from the thermal mass of the frame 1 so that the incident infrared radiation can cause a comparably intense heating and thus a strong signal.
If it may be assumed that the ambient temperature (denoted by the arrows Tu under the frame 1 of the sensor element) is constant it is desirable that the cold contacts 4b are provided above the frame 1 and, if and in as far as it is present there, on the membrane 3. Then the heating of the cold contacts by the impinging infrared radiation is low since the cold contacts are coupled to the thermal mass of the frame 1.
If, on the other hand, it has to be assumed that the ambient temperature may change rapidly the cold contacts as well as the hot contact are preferably not provided above the frame 1 but—as shown in FIG. 1—above the orifice 2 in the frame 1 so that also in so far a disconnection of the cold contacts from the changing ambient temperature is effected. A typical field of application for sensor elements or sensors in changing ambient temperatures is the field air conditioning. Here rapid temperature changes of the ambient temperature may occur so that the sensor and its components may also be subjected to said changes of the ambient temperature.
When the temperature conditions of the cold contact are undefined or change this affects the (indirect) temperature measurement through the (direct) measurement of the infrared radiation carried out in accordance with an object temperature To.
For an exact temperature measurement the dynamic properties of the temperature distribution are relevant. To this end the DE 197 10 946 suggests to configure the heat capacities of the cold spots and the hot spots as well as the heat conductivity in the vicinity of the cold spots and the hot spots in a certain way to be described in more detail. An asymmetric arrangement of the hot spots of the thermopile sensor with respect to the housing is suggested to realise an irradiation of only the hot spots. With the measures known from the cited document it is intended to cause the influence of the ambient temperature conditions on the cold spots to be identical to their influence on the hot spots.
From the EP 1 039 280 an infrared sensor comprising a thermopile sensor is known. The sensor comprises a cap into which a window component is integrated. The window component is attached to the cap by means of plastic from the inside.